Microbes Rule!

Viruses of the Skin

Viruses are well known as obligate parasites and, similar to bacteria, receive much attention when a viral outbreak occurs. From SARS (Severe Acute Respiratory Syndrome), to HIV (Human Immunodeficiency Virus), to Smallpox, viruses are everywhere and are potentially nearly ancient as earth itself. Most viruses are smaller than bacteria, ranging between 20 nm to 300 nm. Mimiviruses are one exception who can be as large as 440nm. Viruses are considered obligate intracellular parasites because they rely so heavily on their host for reproduction of viral structures. Like other microbes such as bacteria, viruses seek the opportunity to reproduce. Finding the right host and complementary cell is important in a virus's ability to deposit its genome, and have it successfully instruct the cell to make virus copies. It has been said that viruses are a bag of genetic material of either DNA or RNA with no concrete organelles unlike eukaryotes with their defined nucleus and mitochondria. Surrounding the virus's genetic material is its capsid made of proteins. Capsids mostly occur in two shapes: cylindrical or spherical, while many bacteriophages (viruses that infect bacteria) have a similar shape to the lunar lander. ³⁵

The life cycle of viruses involves the attachment of the virus to a host cell where it can penetrate the cellular membrane and inject its genes into the cell. Viral genes commandeer the host cell's transcription and translation process to make copies of its proteins necessary for building more viruses. Once the host cell pops open or lysis occurs, new viruses are expelled. The nature of a virus is to take control of its host's protein synthesis machinery in order to reproduce, thus when host cells rupture, viral clones can easily spread to neighboring cells causing eventual tissue damage in a multicellular host such as humans. If the virus is not cleared by its hosts' immune system, latent viral infections may occur. For example the herpes virus and the influenza virus are examples of latent viral infections since it has the ability re-emerge after lying dormant and asymptomatic in their host's tissues. Viruses can infect their hosts in different ways. Passage through the skin is only one mode of entry into the host. ³⁶

In "Human Skin Microbiota" authors Foulogne et al. assert that through the use of high throughout sequencing (HTS) scientists are now able to show the diversity of cutaneous resident or transient viruses. In fact with HTS, more and more scientists are able to produce DNA sequence data that previous attempts could not. The results of these experiments include the identification of new viral strains on the surface of the skin. One of these new strains is the Gyrovirus, which is found in 4% of people with healthy skin. With viral samples taken from healthy and unhealthy individuals Foulogne et al. were able to show the diversity of viruses on the human skin. One result showed the presence of bacteriophages (bacteria infecting viruses) which were present in direct correlation with the type of bacterial species present in the area tested for viral presence. Though very little is known about viruses that are beneficial to humans, research is available that shows that the human skin plays host to bacteriophages known to control Staphylococcus, Pseudomonas, and Propionibacterium species. ³⁷ Exactly how the phages interact with the skin is unknown.

Host – Viral Interactions

Most of what is known of viruses on the human skin is greatly focused on pathogenic viruses. With the advent of technology that is able to collect, culture, and study these microbes, scientists were able to focus their study of viral pathogens simply because non-pathogenic viruses were not as important for study as those causing illness and death in humans. The interactions between human hosts and viruses can be classified into three categories. They are virus-specific, host-specific, and environment-specific. Like with some bacteria, human hosts and their viruses can maintain a homeostatic balance allowing both to exist without hurting each other; however, once there is an imbalance between the two, problems and disease may arise. ³⁸

Lecuit and Eloit question why some viruses make human skin their home. They suggest that viruses may reside on their host so that these become vehicles for future transmission. Their findings lead to further points of inquiry: Could viruses promote keratinocyte proliferation and shedding as a means to transmit their viral genomes? Could further investigations into host-virus relationships lead scientists to find uses for viruses in the production of antimicrobial, anti-inflammatory, and immunosuppressive therapy? ³⁹

Human Papillomaviruses

With high throughout sequencing (HTS) techniques, scientists have found strains of human papillomaviruses (HPV) in the epidermal layers of the skin as well as a Polyomaviridae virus, and Merkel's virus. ⁴⁰ HTS studies of HPV have shown that there are several strains of HPV that typically reside in the skin and shed as part of the skin's normal processes. Areas of the human body where viruses tend to thrive include the hair follicles where Foulogne et al. have shown chronic asymptomatic shedding of HPV. Their presence and eventual shedding from keratinization show little to no tissue damage from the proliferation of HPV. ⁴¹

Papillomaviruses are the culprits of skin diseases like warts. They appear as double-stranded DNA with a cubed capsid. 100 different strains of papillomaviruses are known and the warts that they cause on the epidermal and dermal layers of the skin are host specific. Specifically, the basal squamous epithelia of the epidermis are affected where the virus infects the host cell's cellular processes to increase the rate of cellular division. This increase of cellular division helps the production of viral DNA, viral assembly, and release. ⁴²

Human papillomaviruses are responsible for genital warts associated with the sexually transmitted disease known as HPV infection. Though HPV warts are an obvious skin abrasion, signs of the infection are not always visible. Furthermore, transmission of the virus can occur with contact of an infected skin through unprotected intercourse, oral sex, and bodily fluid exchange. ⁴³ Because HPV infections are asymptomatic in many people, the virus can easily be transmitted and the newly infected have no idea that they have acquired the virus. Surprisingly, 20 million people in the U.S. have HPV ⁴⁴and the majority are in the age range of 15-49 years which tend to be the years that people are the most sexually active.

Variola Virus

A well studied example of virus that has nearly wiped out the native peoples of South America, Mexico, and West Indies is the Variola Virus, which caused smallpox disease. Historically smallpox disease is well documented in ancient Egyptians, ancient Chinese, and of course New World Native Americans, who all suffered from the disease. Because of vaccine-pioneer Edward Jenner's work in the late 1700s and subsequent scientists in the 20 ^th century, smallpox has all been eradicated with the creation of the smallpox vaccine. However, smallpox continues to be an important and pivotal example of the pathogenic potential of a microbe. ⁴⁵

Those affected by the Variola Virus show initial signs of infection with the presence of a fever, lethargy, muscular aches and pains, and the beginnings of a rash on the skin. This rash spreads throughout the body which develops into sores. Once the sores break open, the virus is in its most contagious peak. Over the period of four days the sores become dry and are known as pustules. The skin is raised and protruding. Once a scab develops, new epidermis is generated and will replace the dried pustules and scabs. During the period between the initial appearance of the rash to the scab formation, Variola virus is contagious. Desiccation and shedding of the scab represents the point at which the virus is no longer infectious. ⁴⁶